In a driving method of a display device, there are an active matrix drive and a passive matrix drive mainly. A passive matrix drive has a structure that a display element is sandwiched between electrodes arranged in matrix, which can be manufactured at low cost. However, when one pixel is driven, other pixels cannot be driven so that it is not good for a large-area or high-definition display device. On the other hand, an active matrix drive has an active element and a unit for holding luminance data in each pixel, so that manufacturing cost is higher than a passive matrix drive. However, while one pixel is driven, other pixels can emit light holding luminance data. Therefore, an active matrix driving method is used for most of large-area or high-definition display devices.
An active matrix display device has a unit for holding luminance data in each pixel as described above. The display device can be classified by whether the luminance data has a digital value or an analog value. When the luminance data has a digital value, a light emitting element has only a binary value of on or off, thereby a display image has only two gray scales. A method of expressing multi gray scale by displaying images of binary values quickly and repeatedly is widely used (time gray scale method). In addition, when the luminance data has an analog value, luminance of a display element can be controlled with an intermediate value; therefore, a time gray scale method is not always required in order to express a multi gray scale.
An active matrix drive display device with luminance data having an analog value is mainly, for example, a liquid crystal display. A liquid crystal display has been spread widely, but has problems such as unfitness for displaying a moving image because of slow response speed, and dependence on viewing angle. In addition, a display element is not self-light emitting type; therefore, a back light is required so that power consumption is high. Therefore, development of a new display device replacing a liquid crystal display is expected.
On the other hand, a display device of so-called self-light emitting type, of which a pixel is formed of a light emitting element such as a light emitting diode (LED) attracts attention. As a light emitting element employed for a self-light emitting type display device, an organic light emitting diode (also called OLED (Organic Light Emitting Diode), organic EL element, electroluminescence (EL) element, and the like) attracts attention and is becoming to be used for an EL display and the like. Because a light emitting element such as an OLED is self-light emitting type, a pixel has higher visibility, a back light is not required, and response speed is faster as compared to a liquid crystal display. Therefore, an active matrix drive display device which employs the organic EL element as a display element has been developed actively.
Here, a description is made of an organic EL element. Luminance of an organic EL element is determined by a flowing current value. This nature mainly causes a problem of an organic EL element driven by an active matrix drive. In other words, when a voltage of an analog value is written to a luminance data holding unit (e.g., a capacitor) of a pixel like a liquid crystal display, an active element controlling a current flowing to a display element is controlled in an analog manner, unlike a liquid crystal display in which a voltage applied to a display element is controlled in an analog manner. The active element is provided in each EL element; therefore, a variation in electrical characteristics of an active element in each pixel directly causes a variation in luminance.
Accordingly, when a current drive type display element such as an organic EL element is driven by an active matrix drive by an analog value, it is important to compensate a characteristic variation of an active element which drives a display element. For the method thereof for example, a current input type display element is employed in which a structure of a pixel circuit is devised.
In a current input type pixel circuit, an analog current is employed as luminance data inputted to a pixel. Note that an analog current here refers a current outputted from a circuit which can control a current value by multi-level. An analog current (also referred to as a data current) made by such a peripheral driver circuit corresponding to luminance of a display element is supplied to an active element of each pixel, and a voltage applied to the active element at that time is held. As a result, the current value is held and may keep to be supplied to a display element even after a supply of data current is stopped. FIG. 8 shows an example of such a pixel circuit. A circuit shown in FIG. 8 includes a first power supply line ANODE, a second power supply line CATHODE, a current source for supplying a data current Idata, a wire DATA which the data current Idata flows, a display element 10, a driving transistor Tr1, a capacitor Cs as a luminance data holding unit, a switch Tr2 for connecting and disconnecting between a gate electrode and a drain electrode of the driving transistor Tr1, a switch Tr3 for selecting a pixel in which the Idata is supplied to the driving transistor Tr1, and a switch Tr4 for connecting and disconnecting between the display element 10 and the drain electrode of the driving transistor Tr1.
The current input type pixel circuit can keep supplying a data current as it is regardless of a characteristic of an active element, thereby being suitable for driving a current drive type display element by an active matrix drive. However, when a current value of a display element is very small when driven display element such as an organic EL element, there is a problem in that time (also referred to as program time) for charging the capacitor Cs becomes very long since a data current corresponds one-to-one to a current value of the display element 10 when driven in the circuit shown in FIG. 8.
Therefore, a current input type pixel circuit is suggested in which a data current can be increased against a current value when a display element is driven by adding a capacitor in a pixel circuit (refer to Patent Document 1).
[Patent Document 1]
Japanese Patent Laid-Open No. 2004-310006